1. Field of the Invention
The present invention relates to a light amount adjustment member, for example, a dipping filter in a light amount adjustment device used in a photographing apparatus such as a camera, an optical instrument, or the like, and particularly to a production process of a light amount adjustment member suitable for use in the production of an Neutral Density (“ND”) filter whose spectral transmittance is substantially constant in a visible light region, a light amount adjustment member, and a light amount adjustment device and a photographing apparatus using the light amount adjustment member.
2. Related Background Art
Into an optical instrument such as a digital camera or video camera, a diaphragm device has heretofore been incorporated for the purpose of controlling the light quantity thereof. In this diaphragm device, it is generally conducted to control the light quantity using a diaphragm blade. However, when the diaphragm diameter becomes too small to a high-luminance subject in particular, deterioration of resolving power caused by diffraction occurs. Therefore, the diaphragm diameter is limited, and at the same time a neutral density filter (hereinafter abbreviated as “ND filter”) or the like is used as a light amount adjustment member to limit the quantity of light transmitted, thereby preventing the deterioration of image quality. Specifically, an ND filter that is a separate member from a diaphragm blade is bonded to a part of the diaphragm blade with an adhesive, whereby a diaphragm opening is retained at a certain size without stopping down the diaphragm diameter to a very small diameter when the luminance of a subject is high, and the ND filter is located on an optical axis instead to limit the quantity of light transmitted. In some cases, a filter having a gradient in the light amount adjustment function thereof (hereinafter referred to as “density gradient”) may be used as the ND filter to further control the light quantity by moving this filter on the optical axis. There have also been proposed various diaphragm devices constructed by letting the ND filter independently have an optical action without being installed in the diaphragm blade.
As the ND filter as the light amount adjustment member in such light amount adjustment devices as described above, a metal film or dielectric film formed by vapor deposition or the like, or a multi-layer laminate thereof is generally used. This is attributable to the fact that these materials have good optical properties and excellent durability. Other ND filters include those of the type that a dye or pigment capable of absorbing light is mixed and incorporated into glass, or cellulose acetate, PET or the like that is a material for forming a transparent film, and those of the type that a dye or pigment capable of absorbing light is applied to a transparent base material composed of the above-described material. Further, a production process of an ND film making good use of a silver halide film has also been proposed (see Japanese Patent Application Laid-Open No. 05-173004). The light amount adjustment member having a density gradient is used as a light amount adjustment device for laser beam printers or the like even when its spectral transmission properties are not constant (see Japanese Patent Application Laid-Open No. 11-14923).